1. Field of the Invention
The present invention relates generally to a system for actuating downhole tools using a fluid pressure differential defined in a well as a power source.
2. Description of the Prior Art
The basic function of most downhole tools involves surface manipulation of a downhole operation system to accomplish a task such as opening a valve, for example the opening and closing of a tester valve or a circulation valve.
This process usually involves a power piston which works off a pressure differential acting across a hydraulic area.
There are several ways in which this pressure differential can be achieved.
One technique is the use of a nitrogen charged system in which the nitrogen acts as a spring which supports hydrostatic well annulus pressure, but which can be further compressed with applied pressure at the surface allowing linear actuation across a hydraulic area downhole. An example of such a tool is seen in U.S. Pat. No. 4,711,305 to Ringgenberg.
Another system uses the differential between hydrostatic pressure and an atmospheric air chamber. An example of such a system is seen in U.S. Pat. Nos. 4,896,722; 4,915,168; 4,796,699 and 4,856,595 to Upchurch.
Yet another system provides first and second pressure conducting passages from either side of the power piston to the well annulus. A metering orifice type of retarding means is disposed in the second pressure conducting passage for providing a time delay in communication of changes in well annulus pressure to the second side of the power piston. Accordingly, a rapid increase or rapid decrease in well annulus pressure causes a temporary pressure differential across the piston which moves the piston. An example of such a system is seen in U.S. Pat. No. 4,422,506 to Beck.
Still another approach is to provide both high and low pressure sources within the tool itself by providing a pressurized hydraulic fluid supply and an essentially atmospheric pressure dump chamber. Such an approach is seen in U.S. Pat. No. 4,375,239 to Barrington et al.
There are limitations inherent in many of these designs. Those tools which use either a high pressure source or low pressure reference defined by a fixed volume within the tool itself are typically limited in the number of operating strokes they can provide since they either run out of pressurized high pressure fluid, or run out of space in the low pressure reference zone.
Those systems like the Beck U.S. Pat. No. 4,422,506 which utilize a time delay in communication of well annulus pressure changes to one side of the piston can provide an unlimited number of operating strokes in many cases, but they do have the inherent drawback of the time delays which are necessary between operating strokes.
The prior art also includes tools which have operated in response to a pressure differential between a well annulus and the interior of a tubing string, but these tools have not been capable of repeated operation. Examples of such tools are found in U.S. Pat. No. 3,779,263 to Edwards et al.